1. Field of the Invention
The present invention relates to a semiconductor device and particularly to a thin film semiconductor device, a method of making the same and a liquid crystal display device using the same device.
2. Description of the Related Art
As the base material on which a thin film semiconductor device (a thin film transistor (TFT) principally used in an image display device) is formed, a high-temperature polycrystalline Si (silicon) has been mainly used. In this material, the polycrystalline Si (silicon) is formed on a quartz substrate through the high-temperature heat treatment under the temperature of about 900° C. and thereby the polycrystal Si of comparatively large grain size (200 to 500 nm) has been formed. A TFT formed on this high-temperature polycrystal Si uses a Si thin film having lower density of grain boundary and good crystallinity as the channel region and therefore it is possible to obtain the electron mobility of 100 to 150 cm2/(V·s) which is approximated to that (about 500 cm2/(V·s) of single crystal. However, since this high-temperature polycrystal Si is required to use an expensive quartz substrate that is resistive to high-temperature process, substrate cost makes it difficult to reduce the total cost of the device and thereby wide use of TFT has been restricted.
In these years, a low-temperature polycrystal Si has been investigated in place of such high temperature polycrystal Si. In the case of low-temperature polycrystal Si, amorphous silicon or micro crystalline silicon formed on a low cost glass substrate with the plasma CVD method or the like is crystallized with the melt-grown method such as excimer laser anneal. This method brings a merit that TFT can be attained at a very low cost because the polycrystalline Si thin film can be formed at the temperature lower than the glass softening temperature of about 450° C. However, the existing low-temperature polycrystalline Si has the smaller grain size than that of the high-temperature polycrystalline Si. Therefore the TFT using the low-temperature Si as the element material results in large carrier scattering at the grain boundary and has the electron mobility restricted to about 30 to 50 cm2/(V·s). Such a small electron mobility cannot attain the required element velocity and therefore results in a problem that the elements that can be formed on a sheet of glass substrate are restricted in the kinds thereof. For instance, in the case of the image display apparatus, the matrix element may be formed on the glass but the peripheral circuits such as the other source driver, gate driver, shift register and peripheral controller are formed on the printed circuit board and this board must be used through connection to the glass substrate with cable terminals. The method explained above has the problems that the display size is reduced (4-inch to 10-inch) and total cost of the apparatus becomes high.
On the other hand, recently, various techniques have been proposed to allow large grain size for the low-temperature polycrystalline Si and control the position of the crystal grain. For example, the technique (Japanese Unexamined Patent Publication No. H8-316485) to form seed crystal constituted of island type pattern on an insulator substrate and to realize solid-phase crystallization of amorphous Si on such core, the method (H8-31749) to form a deposited layer of amorphous Si on polycrystalline-Si and convert the polycrystalline-Si exposed at the surface to the next seed crystal, the method (H10-55960) to selectively make amorphous layer from the partially crystallized Si thin-film with ion implantation and then realize crystal growth again using the remaining crystal part as the nuclei, the method (H9-27452) to disperse metal element that accelerate crystallization of Si into the amorphous Si film under a high-temperature condition to crystallize the amorphous Si film, the method (H10-97993) to change in the step-like manner the fluence and pulse width of laser anneal, and the method (H8-288515) to form a first amorphous Si film on the insulated substrate forming the step areas, to form a first polycrystal-Si film having uniform orientation at the surface of step area side through the heat treatment of 24 hours, thereafter to conduct again the heat treatment of 24 hours for the second amorphous Si film formed thereon in order to form the second polycrystal-Si film having large gain size that is controlled in both alignment and crystal gain boundary.
However, although various attempts have been conducted, the crystallization method explained above to attain large crystal grain size still cannot obtain the low-temperature polycrystal-Si having good crystallinity with good reproducibility and various problems are yet left unsolved for mass-production. For instance, the crystal grain size is increased but fluctuation of characteristic among TFT elements due to the position deviation of crystal grain cannot be controlled. Moreover, surface orientation of poly-crystal is formed at random and therefore here rises a problem that electron mobility depending on the surface alignment is fluctuated among the TFT elements. Accordingly, large influence is applied to the manufacturing yield of a large-scale thin-film semiconductor integrated circuit device formed by integrating many pieces of TFT. Particularly, it is very difficult to adopt such TFT to mass-production of a liquid-crystal display device having a large display area. Moreover, the existing crystallization process to control the alignment and position of grain boundary requires the long-term process. Therefore, mass-production thereof is always accompanied with increase of manufacturing processes, fluctuation of characteristic and drop of manufacturing yield, etc. and these are serious problems for realization of the liquid-crystal display device having a large display area of 15-inch or more.